Not applicable.
The invention relates to a method for measuring the temperature and moisture of a web.
In paper machines and the like in which a continuous material web is manufactured, it is necessary to measure the temperature and moisture of the material web in different parts of the process.
In the arrangements of the prior art, the temperature and moisture of the web are generally determined by means of a measuring head traversing across the web in a cross direction. In this kind of arrangement, measurement takes place at different times across the width of the web and one traversing cycle typically takes 20-40 seconds.
Commonly assigned FI patent 80522 discloses a method and an apparatus for measuring the tension of a web. The tension of the moving web is measured by placing close to the web a measurement rib which has a curved surface in the running direction of the web and in which pressure measurement detectors have been placed in holes of the rib. An air cushion is formed between the moving web and the curved surface, the air pressure in the air cushion being proportional to the tension of the web. The tension of the web is measured indirectly by measuring the pressure of this air cushion. In the method, the pressure difference between a reference pressure and the pressure of the air cushion is measured. The pressure of the air surrounding the apparatus can be used as the reference pressure or the reference pressure can be produced by a separate device.
In the method according to the invention, the apparatus disclosed in the above-mentioned FI patent 80522 can be applied in the measurement of the temperature and moisture of the web.
The invention is based on the fact that the moisture and temperature of a web are measured indirectly from an air cushion formed between the web and a surface which is at least partly curved in the running direction of the web, at the same time at several measurement points spaced from one another in the cross direction of the web. With respect to moisture, it is relative humidity that is primarily measured in the method. The value of local moisture at the point of measurement is primarily determined by the moisture of paper. The phenomenon is based on the fact that humidity tends to be distributed uniformly in the ambient air. The fast-moving web causes a strong air vortex at the points of measurement, with the result that the relative humidity of a small air space in front of a detector changes very quickly in a manner proportional to the moisture of the web moving across the measurement point.
The method according to the invention for measuring the temperature and moisture of a web is simple, reliable and accurate, and it can be employed in any part of a paper machine, such as, for example, in connection with a press section, a dryer section, a coating device, and a calender. Measurement can be performed before said section/device and/or after said section/device and/or inside said section/device.
The temperature and moisture measurement signals from each detector are passed to a monitoring system of a paper machine, which includes a computer program for computing and monitoring moisture and temperature profiles. The moisture and temperature profiles can then be used for control of various parts of the process.